Text 862, 135 rader
Skriven 2004-11-19 06:15:00 av Guy Hoelzer (1:278/230)
Ärende: Re: The "fuel" of evoluti
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Hi Jim,

in article cnhhln$1rrr$1@darwin.ediacara.org, Perplexed in Peoria at
jimmenegay@sbcglobal.net wrote on 11/17/04 11:06 PM:

> We all know that heritable variation in characters affecting fitness
> is a prerequisite for natural selection.  Metaphorically speaking,
> we can even say that heritable variation is the "driving force"
> for NS.
> 
> This is a good metaphor because variation plays a force-like role.
> Assuming you quantify "variation" as the variance in fitness, the
> infamous "Fisher's Fundamental Theorem" assures us that the
> magnitude of the response is proportional to the magnitude of the
> "force".  Furthermore - (correct me if I am wrong here!) - the
> direction of the response matches the direction of the "force".
> If the total variance in fitness is divided among several
> characters, the characters that evolve fastest are those that make
> the largest contribution to the fitness variance.

I like your description of the currently dominant metaphor.
 
> My question is whether a different metaphor might not also be
> good.  Suppose we were to say that heritable variation is the
> "fuel" of evolution.  Can we come up with a way of quantifying
> "variation" so as to make this a good metaphor.

And I prefer the new metaphor you are considering.  I don't see, however,
why standard statistical estimators of heritable variation would be
insufficient.
 
> For the "fuel" metaphor to be a good one, we need to quantify
> "variation" to make it fuel-like.  That is, variation must be
> inevitably "consumed" by natural selection.  Furthermore, we
> want the amount of the response to be roughly proportional to
> the amount of fuel consumed - not, as in the "force" case, to
> the amount present.

Indeed.  This would be a real improvement of the "fuel" metaphor over the
traditional "force" metaphor.

> Can this metaphor be made to work and to
> work well?  (Of course, nothing rules out that the "fuel" may
> be replenished by mutation or by some other process outside of
> natural selection proper.)

Clearly there is no problem with accounting for the replenishment or mining
of "fuel" in the new metaphor.  One way in which the "fuel" metaphor might
break down is that the functional integrity of machines generally terminates
when the rate of fuel supply drops below some critical threshold.  Your
car's engine shuts down, for example, and does not start running
automatically when you put more fuel in the tank.  In other words, once you
interrupt the temporal coherence of fuel processing by a machine it takes
more than fuel to re-instantiate the process.  This is seemingly not the
case for natural selection, which appears to happen automatically in the
presence of "fuel" (heritable variation for fitness).
 
> Frankly, I don't know.  I'm pretty sure that the metaphor does
> NOT work if you stick to variance in fitness as your quantification
> scheme for "variation".

Why?  Wouldn't you agree that heritable variation in any trait other than
fitness could not serve as fuel for natural selection.  I'm not sure how we
should think about the interaction between non-heritable fitness variation
and natural selection under the paradigm of the "fuel" metaphor.  It might
be helpful here to consider the process of natural selection as "feeding on"
all kinds of inherent fitness differences, but only affecting evolution
through the heritable components of fitness variation.  Notice that this
line of thought has taken me back to Darwin's view of fitness as the
inherent functional quality of an organism (more generally, a reproductive
entity to allow for multilevel selection), and away from the empirical
estimators of fitness differences derived under the neo-Darwinian paradigm.
In other words, it might be better to think of fitness as an expected value,
or a potential, rather than something that can be determined retrospectively
(e.g., with data on realized survival and reproduction).  This shift in the
meaning of "fitness" would still allow for empirical estimation of
fitnesses, but we would have to recognize that the error in these estimates
would come from both our own sampling and from natural sources of
stochasticity.  In fact, this approach to fitness estimation might be a nice
way to model the interplay between drift and selection.  I must admit,
however, that I don't have a more concrete suggestion at the moment as to
how to quantitatively incorporate natural stochasticity into the fitness
model.

> But does some other quantification work?
> Has anything along these lines been published?

I haven't seen it.
 
> I was led to this line of thought a couple months ago while discussing
> DS Wilson's use of the Price equations in support of "trait group
> selection".  I made the point that the group selection process is
> driven by inter-group variation, but that the process itself must
> inevitably decrease ("consume?") the amount of inter-group variation.
> Hence, I was led to say that the process must quickly "run out of
> gas" unless the "fuel supply" is replenished by some other process -
> assortive group formation, for example.  That is what I was thinking
> at the time, but I still don't know whether my metaphor is a good
> one and whether it applies to other situations.  What do you all
> think?
> 
> Of course, the true value of a metaphor lies in the follow-up
> questions that it suggests.  Is there a difference in the fuel
> efficiency of various evolutionary situations?

Great question.  One answer coming from the traditional paradigm is that
fuel efficiency is worse in smaller populations.  However, this issue might
be accounted for with the alternative fitness model I suggested above.
Natural stochasticity would effectively blur (add error to) the values of
inherent fitnesses, thus reducing the distinctions between them.  Because
the "fuel" would be variation in inherent fitness differences, blurring the
distinctions would also reduce the amount of fuel, rather than reducing the
"fuel efficiency."  The forms and extents of epistasis and pleiotropy might
affect "fuel efficiency."

> Does the combustion
> require two different fuel components (oxidizer and reductant)
> with either component potentially limiting?  Etc.

At first glance this question seems to push the metaphor too far.  I'll be
curious to see if you or anyone else can think of an analogy for the
components of combustion.

Regards,

Guy
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